Study population: The study population is composed out of people qualifying for a surgical weight loss procedure (RYGB or LAGB). A total of 45 obese (BMI > 35 ≤ 45) study participants between age 18 and 60 will be investigated Intervention: One subject group (n=15) will undergo a diet intervention, which consists of a very low calorie diet (VLCD; Modifast) containing 2.1MJ/d for 1 to 2 months, until they lost 10% of their initial body weight.